help understand the TL431

[jarthel]

I was reading the TI datasheet before but someone pointed out the onsemi one (http://www.onsemi.com/pub/Collateral/TL431-D.PDF) which is frankly better. but of course I still have a question (or else I do not have to create a thread )
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I initially thought that the R1 and R2 mentioned in the datasheet must be valued so that the voltage across R2 to ground is 2.5V. I got this idea upon reading an article on tubecad.

But when I looked at onsemi's datasheet (figure 18), it seems my initial understanding of R1 and R2 is incorrect. looking at the formula in figure 18, I can arbitrarily choose any value for R1 and R2 and do not have to worry about the 2.5V reference.

Is this due to the fact that the tl431's current load on the reference pin varies to maintain 2.5V across R2?

example:

I arbitrarily chose 2.2K as R2. so the tl431 current load on the reference pin will be adjusted to 2.5/2200. is this correct?
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thank you.

[peranders]

You have always 2.5 volts across R1 but you avoid trouble if you choose rather low resistor values and 2.2 k is "low".

Just use the formula in page 5, fig 2.

The rule is that current at the REF pin should be small compared the one in the R1 and R2.

I can help you.

Incoming voltage, min, max?

Max desired output current(=the load)?

Desired output voltage?

[jarthel]

tL431 is part of this circuit: http://www.raleighaudio.com/ccs.htm

I have a feeling the tube is there because the tubes takes the brunt of the current load as the tl431 is limited to 100mA. wild guess but I'm sure someone will correct me

input = around 170VDC
output = 150VDC
desired output current = 70mA

[davidsrsb]

No, the current through the tube and the TL431 is the same, the tube is acting in cascode and is there to handle the high voltage.

[jarthel]

Originally posted by davidsrsb
No, the current through the tube and the TL431 is the same, the tube is acting in cascode and is there to handle the high voltage.

thank you.

[jackinnj]

the TL431 is one of those "ubiquitous" devices which is both elegant and cheap -- the ti website has a very good model which incorporates the tempco factors in the device.

remember, this is a voltage reference with an error amplifier and good temperature compensation.

[hailteflon]

This forum's administration requires that I (new member) make a few replies before I am certified as not a bringer of flame-o-grams.

Since I am a big fan of the 431.................

The TL431 is a very good device. It is also a current source/sink. It is ideal for biasing the output stages of AB amps and the tail current of a differential input pair.

It is a programmable zener, with 2.5V as the reference/minimum voltage. Unlike a zener diode it is very quiet and has good dynamic impedance. Dynamic impedance sounds to me to mean that it reacts faithfully to changes.

It can also be used to make a good adjustable voltage regulator.

They are quiet and stable. I recall that National makes them. Most manufacturers have pretty much the same data sheets, but National may add something.

Another cheap and excellent TO-92 device is the LM335 temperature sensor. LM335AZ is 84 cents from Mouser. Regards, Mark